(1) Field of the Invention
The present invention relates to a technology for controlling an optical fiber amplifier used in an optical transmission system applied with a wavelength division multiplexing (hereafter referred to as WDM) transmission technology, and in particular, relates to a control method of a pumping light in a rare-earth element doped optical fiber amplifiers or a Raman amplifier, and to an optical transmission system using this method.
(2) Description of the Related Art
Recently, the construction of optical transmission systems or photonic networks, which realize high-capacity optical communications by applying a WDM transmission technology, has become in practical use. In such optical transmission systems, it is possible to collectively amplify a WDM signal light using for example an optical fiber amplifier, such as a rare-earth element doped optical fiber amplifier, a Raman amplifier or the like, thereby repeatedly transmitting the WDM signal light over a long distance.
For the above described optical transmission systems or the like utilizing the optical fiber amplifier, there are common problems, for example, a demand for extending the WDM signal light wavelength band to increase the transmission capacity, or the improvement of an optical signal-to-noise ratio (OSNR) in each repeating interval to realize a superior transmission characteristic.
As a conventional technology for extending a signal light band, in a system as shown in (A) of FIG. 15 for example, wherein a WDM signal light in a wavelength band of 1530 nm to 1565 nm, generally referred to as a C-band, is repeatedly transmitted on a transmission path fiber 3 from a transmission terminal (Tx) 1 to a reception terminal (Rx) 2 while being amplified by a plurality of repeaters 4 each provided with a rare-earth element doped optical fiber amplifier corresponding to the C-band, there has been known a technology as shown in (B) of FIG. 15, wherein a rare-earth element doped optical fiber amplifier 4L corresponding to a wavelength band of 1565 nm to 1625 nm, generally referred to as an L-band is disposed in parallel with a C-band rare-earth element doped optical fiber amplifier 4C of each repeater 4, to extend the signal light band into both the C-band and the L-band. Further, in a system utilizing for example a Raman amplifier, a technology has also been known wherein Raman pumping light source is added, to give a plurality of pumping lights having different wavelengths to an amplification medium, thereby realizing the extension of the signal light band (refer to Japanese Unexamined Patent Publication No. 10-73852, Japanese Unexamined Patent Publication No. 2000-98433, Japanese Unexamined Patent Publication No. 2002-76482, Japanese Unexamined Patent Publication No. 2002-303896, and the literature by M. Takeda et al., “Active gain-tilt equalization by Preferentially 1.43 μm—or 1.48 μm—Pumped Raman Amplification”, ThA3, OMA 1999).
As a conventional technology for improving the OSNR in each repeating interval, in a system wherein a plurality of repeaters each using a rare-earth element doped optical fiber amplifier is arranged on a transmission path fiber, a technology has been known wherein a distributed Raman amplifier (DRA) which uses the transmission path fiber in each repeating interval as an amplification medium, is applied to increase the power of the WDM signal light input to the repeater from the transmission path fiber by the Raman amplification as shown by the dotted line in a level diagram in FIG. 16 for example, thereby improving the OSNR. Generally, the DRA applied to such a system is of a backward pumping type configuration in which a pumping light is given to the transmission path fiber in a direction opposite to a propagation direction of the signal light. Moreover, in order to further improve the OSNR in the above system, a system applied with a forward pumping type Raman amplifier in which a pumping light is given to the transmission path fiber in the same direction as the propagation direction of the signal light, is also under investigation. However, for the forward pumping type Raman amplifier, there have been known a problem of RIN transfer in that a relative intensity noise (RIN) in the pumping light transits into the signal light as the noise, and a problem of PDG (polarization dependent gain) in that a Raman gain significantly depends on a signal light polarization state, (refer to the literature by C. R. S. Fludger et al., “Pump to Signal RIN Transfer in Raman Fiber Amplifier”, Journal of Lightwave Technology Vol. 19, No. 8, 2001, and the literature by J. Zhang et al., “Dependence of Raman Polarization Dependent Gain on Pump Degree of Polarization at High Gain Levels”, OMB4, OAA 2001). In order to reduce the problems of the above forward pumping type Raman amplifier, the utilization of a bidirectional pumping Raman amplifier applied with both the forward pumping and the backward pumping is in the study (refer to the literature by J. Bromage et al., “Raman-enhanced pump-signal four-wave mixing in bidirectionally-pumped Raman amplifiers” OWA5, OM 2002).
However, the conventional technology for extending the signal light band and the conventional technology for improving the OSNR in each repeating interval, cause the following problems.
That is to say, as shown in FIG. 15, in the system using the rare-earth element doped optical fiber amplifiers, when the rare-earth element doped optical fiber amplifiers corresponding to different bands are added in order to extend the signal light band, repeaters equivalent to those prior to the addition are added in parallel. Therefore, there is caused a problem of high cost in band extension service for when transmission capacity demands are increased after the installation of system for example. Further, also when the extension of signal light band is realized by adding the pumping light sources in a system using Raman amplifiers, high-power pumping light sources are generally expensive, resulting in an increase in band extension service cost. In order to provide the band extension service at a minimum cost, it is desirable to be able to readily extend an amplification band of the rare-earth element doped optical fiber amplifier used in a large number of optical transmission systems.
Moreover, when the above bidirectional pumping type Raman amplifier is utilized to improve the OSNR, there is caused a problem in that the performance of the Raman amplifier is considerably varied depending on a position of lump loss existing on the transmission path fiber being the amplification medium. Specifically, this lump loss is a comparatively large loss, which occurs in a concentrated manner due to the bad connection of a connector or a fused portion connecting between transmission path fibers, the bending of fiber or the like.
For example, when the comparison is made on the case where 1 dB lump loss exists on an input end of a signal light and the case where 1 dB lump loss exists on an output end of the signal light, in a 100 km transmission path fiber, as shown in FIG. 17, a signal light level diagram differs considerably between the two cases. Therefore, when the lump loss exists on the input end, the level diagram is lowered over the entire repeating interval, to deteriorate the OSNR. On the other hand, when the lump loss exists on the output end, there is a possibility that the power of the signal light in the transmission path fiber is increased, and thus various non-linear phenomena occur, to deteriorate the signal waveform.
If a large margin to the lump loss is obtained, considering the deterioration in the performance of the Raman amplifier occurring due to the above lump loss, the cost of the bidirectional pumping type Raman amplifier is increased. It is therefore desirable to suppress a variation in the performance of the Raman amplifier due to an influence of lump loss as much as possible.